Powder injection molding (PIM) processes offer the possibility of molding complex net-shape components with high precision using highly automated equipment. It is anticipated that as in the plastic industry, automated injection molding of high precision inorganic components, such as ceramic, metals, cermets and intermetallics would make it cost-effective and competitive. Injection molding of particulate inorganic materials occurs in a manner analogous to injection molding of plastics. A granular precursor material composed of ceramic or metal powder dispersed in a thermoplastic organic binder system is heated until it softens and is forced into a mold cavity under high pressure such as 30 MPa or higher and cooled. The organic binder is then removed at a relatively low temperature by a process commonly known as "debinding", and the debinded components are sintered at a relatively higher temperature. Shrinkage of the thermoplastic binder can lead to internal defects in the molded part. The debinding process is generally slow and can take as long as several days. Binder removal can also generate defects such as deformation and voids and cracks in the sintered part. The high pressure along with abrasivity of the ceramic and other inorganic particles contribute to rapid tool wear in the PIM process.
Since the advent of popularity of high performance ceramics, considerable efforts have been expended to come up with a viable ceramic powder injection molding (CPIM) process which may be applied to all ceramic powders regardless of particle size, however, despite considerable investment of time and money by various research communities all over the world, CPIM has failed to deliver the desirable end products. The first and foremost reason for this is that the productivity of CPIM is not as high as anticipated. While it may take only a few seconds or a minute to mold a component, the total production time can be a week or longer. Also, defect distributions characteristic of CPIM limit the reliability of molded components as compared to other conventional ceramic forming processes. As a result, CPIM is currently used primarily to manufacture non-critical components such as textile thread guides, investment casting cores, and coarse refractories. The use of CPIM to manufacture high performance and reliable ceramic components having high productivity is still not possible.
The direct coagulation casting (DCC) process involves the coagulation of an electrostatically stabilized ceramic slurry. An in situ enzyme catalyzed reaction is used to change the pH or increase the ionic strength of the slurry to coagulate the slurry and cause it to "set". Remarkable Weibull moduli have been reported, which indicates that a very uniform and high-quality microstructure is achieved rendering the molded components more reliable than before. This process seemingly meets many of the requirements for an improved forming process except for difficulties in controlling the reaction time. The window for gelation time is in terms of only several minutes which makes it impractical to implement the DCC as a production injection molding process.
DCC has been recently described by T. J. Graule, W. Si, F. H. Baader and L. J. Gauckler of the Swiss Federal Institute of Technology, "Direct Coagulation Casting: Fundamentals of a New Forming Process for Ceramics," pp. 457-461 in Ceramic Transactions Vol. 51: Ceramic Processing Science and Technology. Edited by Hans Hausner, Gary L. Messing and Shin-ichi Hirano. American Ceramic Society, Westerville, Ohio, 1995.
Binder Coagulation Casting (BCC) is another process for molding ceramics which is described in U.S. patent application Ser. No. 08/931,174 filed Sep. 16, 1997. In the BCC process an aqueous ceramic slurry containing polyelectrolyte deflocculants such as polycarboxylic acid and high molecular weight binder is gelled in a controlled fashion by the action of a chemical initiator and/or by increasing the temperature of the slurry. The mechanism by which gelation occurs is the coagulation of dispersed ceramic particles by high molecular weight binder molecules as the level of adsorption and conformation of polymer molecules change as the pH of the system changes.
Obviously, there are numerous potential applications of ceramic components that would become technically and economically feasible if there was a more productive forming technique than the current CPIM that could produce reliable complex-shaped components with fewer defects. The ideal forming technique must be able to produce complex shapes at high rates (less than a minute per part) and high yield (&gt;90%) with excellent dimensional control and minimal internal defects. The total manufacturing cycle should be reduced to few days instead of few weeks.